The invention of the present application relates to a variable valve actuation (VVA) apparatus for an internal combustion engine, which controls the opening/closing timing of intake and exhaust valves in accordance with the engine operating conditions.
A typical VVA apparatus for an internal combustion engine is disclosed in Japanese document P2001-329811A. The VVA apparatus comprises phase alteration devices arranged at the front ends of intake and exhaust camshafts, respectively, to which torque of a crankshaft is transmitted through a chain. The intake-side phase alteration device comprises in a housing a drive-input sprocket or rotator for directly transmitting thereto torque of the crankshaft through the chain and an intake-side transmission sprocket or rotator which is a member separate and distinct from the drive-input sprocket. On the other hand, the exhaust-side phase alteration device comprises in a housing an exhaust-side transmission sprocket or rotator having the same outer diameter as that of the intake-side transmission sprocket. The chain is looped between the two transmission sprockets.
Since the camshaft of the four-cycle engine rotates at the 1:2 ratio with respect to the rpm of the crankshaft, the outer diameter (number of teeth) of the drive-input sprocket of the phase alteration device is determined by the outer diameter (number of teeth) of a crankshaft-side sprocket. Thus, the outer diameter of the drive-input sprocket cannot be reduced freely, whereas the size of the two transmission sprockets can be reduced up to a point by simply setting the outer diameters (numbers of teeth) to the same-value. Therefore, when adopting the above VVA apparatus, even with engine layout having the intake and exhaust camshafts disposed relatively close to each other, torque of the crankshaft can be transmitted to the intake-side and exhaust-side phase alteration devices.
A vane rotor integrated with the exhaust camshaft is relatively rotatably accommodated in the housing of the exhaust-side phase alteration device wherein the transmission sprocket is arranged on the outer periphery. The vane rotor comprises vanes protuberantly arranged on the outer periphery to divide an inside space of the housing into advance and retard chambers. Selective supply/discharge of working fluid from the advance and retard chambers is carried out suitably to produce relative rotation between the housing and the vane rotor. With such so-called vane-type phase alteration device, however, the vane rotor is often put back to its retard position by alternating torque of the camshaft at engine stop or start where the pressure of working fluid is low. Thus, application of the phase alteration device to the exhaust side as-is can impair smooth engine start. Therefore, in the VVA apparatus, a spiral spring serving as a return spring is arranged at the front end of the exhaust-side phase alteration device to bias the vane rotor to its advance position, whereby the vane rotor is put back to its advance position by a biasing force of the spiral spring at engine stop or start.